Semiconductor memory is an enabling component of almost all state-machine and processor-based electronic devices: TV's, computing systems, cell phones, pacemakers, coffee makers, and thousands of other devices. Semiconductor memory serves as an electronic scratchpad for controllers and processors of all sorts—enabling the storage and/or retrieval of information in binary form, which has either a temporarily persistence, i.e., retaining content while energy is available or a more permanent persistence by retaining content during periods when energy is not available. Static and dynamic random access memory, SRAM and DRAM respectively, are examples of memory in the first category, while flash memory, ROM, PROM, EPROM, MRAM, and FRAM memory are examples of memory in the second category (often called nonvolatile memory). Many devices utilize a combination of memory in both categories.
SRAM and DRAM memory are complex devices composed of memory cells to hold bits and ancillary circuits to read, write, and maintain the bits in the cells. Each cell contains one bit of information. An SRAM memory cell is often a configuration of transistors that can store a bit of information, while a DRAM memory cell may contain a semiconductor structure that can hold charge, commonly a capacitor but other structures may also be used to store a charge. The presence of a charge represents a bit value (1 or 0) and the absence of a charge represents an inverse value. A DRAM cell is functionally different from an SRAM cell in that a charge in a DRAM cell must be restored periodically by reading and rewriting the cell as the charge leaks away over time. This complication is offset by the fact that DRAM cells can be made much smaller than SRAM cells and thus can be made with much higher densities. A SRAM, on the other hand, is faster than a DRAM and uses less peripheral circuitry. A SRAM cell is a transistor circuit that can be in two states, each state representing a bit value. A SRAM cell maintains a state that represents a value of a bit as long as energy is available or until the state is changed by ancillary circuitry during a write operation.
Techniques to increase the density of SRAM's are an active area of development. To increase density, the dimensions of the semiconductor structures in SRAM's are often decreased, which decreases allowable mask alignment and other tolerances associated with photolithography during a manufacturing process and consequently increases the opportunities for defects. To ameliorate the effect of defects, spare cells are often incorporated into a memory and substituted for defective cells as needed during a test procedure following the manufacturing process. While this technique may increase the yield, the potential current load of the additional circuitry to effect a substitution of good cells for defective cells, and the potential current load imposed by defective cells remaining in the memory after being functionally replaced may be detrimental. An increased load on ancillary driver circuits can slow the operation of the circuits, increasing access time. Also energy consumption may increase, resulting in higher energy and cooling costs, higher temperatures, and decreased reliability.